Answer:
There are 0,6 moles of helium.
Explanation:
We use the ideal gas formula, we use the gas constant R = 0.082 l atm / K mol and we solve for n (number of moles) of the formula:
PV=nRT --> n= PV/RT
n= 1,20 atm x 12,9 L/ 0.082 l atm / K mol x 315 K= 0,599303135 moles